The design and development of modern commercial and military aircraft usually involves modeling the aircraft under development and determining the performance of the model within a wind tunnel. Typically, the performance of the model is determined by measuring the aerodynamic forces such as lift imparted to the model by an airstream established and maintained by the wind tunnel. In those cases where the model is provided with a "powered nacelle" (a nacelle which houses an air powered, combustorless gas turbine engine simulator), the thrust of the airstream exhausted from the engine model may be utilized in determining that performance parameter known in the art as "net thrust-minus-drag".
Such measurements of lift, and thrust-minus-drag are typically made on force balances, forces on the model being transmitted to the balance by, for example, the structure supporting the model within the wind tunnel. Such performance measurements generally require measurements of airflow as well, such airflow measurements being made with flowmeters, pressure measuring instrumentation, or analogous equipment. It is generally accepted by those skilled in the art that such balances and flow measuring instrumentation have various inherent inaccuracies associated therewith which are manifested as both data bias and scatter. Moreover, such inaccuracies vary from one wind tunnel facility to another whereby testing the same model in different wind tunnel facilities will usually yield disparities in measured performance data. It will be readily apparent that such inaccuracies (as those inherent in the instrumentation per se and those associated with the connection of the model to the wind tunnel) not only have a generally deleterious effect on the overall accuracy of measured data but render meaningful conclusions as to the effects of adjustments in model geometries on model performance, difficult if not impossible to draw.
The above-noted inaccuracies associated with wind tunnel facility instrumentation depend in large measure, on both the magnitude of the airflow through the wind tunnel as the model is tested and the magnitude of the airflow which powers the cumbustorless gas turbine engine simulator. Therefore, since aircraft models are typically tested over a substantial range of flows through both the wind tunnel and the gas turbine engine simulator, it is necessary for obtaining accurate performance data that the inaccuracies associated with the wind tunnel testing be determinable for the particular airflows at which the model is being tested.
Accordingly, among the objects of the present invention is the provision of a method and apparatus for determining the inaccuracies (data bias and scatter) associated with wind tunnel testing of aeronautical models with powered nacelles at prescribed airflow conditions.